1. Field of the Invention
The present invention relates to a beam recorder which records an image on a record medium by a beam.
2. Related Background Art
In a conventional laser beam printer of this type, a rotating polygon mirror for scanning a laser beam is constructed such that all planes thereof scan the beam for forming an image. Accordingly, for each of the planes of the rotating polygon mirror, it is necessary to attain high precision in planarity and parallelism of the planes. However, in actual, use since it is not possible to completely eliminate a difference between parallelisms of the planes, an optical system for compensating resulting ununiformity in the scan in a sub-scan direction (a correcting cylindrical lens) is required. As a result, the apparatus is complex and expensive in view of pat precision, number of parts, and assembly work of the parts. In a low speed and low grade apparatus, the number of planes of the rotating polygon mirror is reduced as much as possible to reduce a cost. Nevertheless, the complex and expensive construction has not yet been resolved.
In the conventional laser beam printer of this type, various restrictions are imposed when an intensity of the laser beam is to be controlled. A semiconductor laser is usually used as a laser light source for the laser beam printer of this type. The semiconductor laser has a disadvantage of variation of intensity with a temperature and reduction of intensity by deterioration. As a result, an intensity control system is adopted in which an output of a photo-sensor mounted in the semiconductor laser is detected to keep the laser light intensity constant.
FIG. 4 shows a laser light intensity control circuit. A microprocessor (MPU) 21 produces a laser on (LON) signal to turn on a transistor 27 and supplies a digital value to a D/A converter 22 to gradually increase the output of the D/A converter 22. As a result, a current I.sub.OP flowing through a laser diode 29 of a laser unit 28 gradually increases by a constant current circuit (CC) 24 so that a light intensity of the laser diode 29 increases. A laser beam from the laser diode 29 is emitted externally and also to an internal photo-diode 30. As a result, a photo-diode current I.sub.M which is proportional to the light intensity emitted externally flows. It is converted to a voltage V.sub.M by a resistor 31 and supplied to an amplifier (AMP) 25. The output of the amplifier 25 is supplied to an A/D converter 23 where it is converted to a digital signal, which is read by a microprocessor 21. The microprocessor 21 increases the output of the D/A converter 22 until the output read by the microprocessor 21 reaches a predetermined value, when the output of the D/A converter 22 is fixed. The digital value currently applied to the D/A converter 22 is stored and the signal LON is turned off. In this manner, the laser light intensity is controlled.
In a print mode, the microprocessor 21 applies the stored digital value to the D/A converter 22. A video (VDO) signal turns on and off a transistor 27 so that the laser diode 29 is turned on and off by a constant current pulse.
In the laser light intensity control system described above, it is necessary to continuously emit the laser beam when the light intensity of the laser is detected. Where the conventional rotating polygon mirror is used, the emitted laser beam is necessarily directed to a photosensitive member. Accordingly, if toner deposits to the photo-sensitive member, the toner is wasted. Accordingly, it is necessary to control the process such that the toner is not developed. In a one-sheet print operation, the process may be controlled prior to the actual print operation such that the toner is not developed and then the light intensity of the laser may be detected. In continuous print operation, because of the change of laser intensity by temperature, it is necessary to detect the laser light intensity between n-th printing and (n+1)th printing. Accordingly, in order to control the process such that the toner is not developed during that period, a complex process control is required, and fast response of the development process is required.
In the conventional laser beam printer of this type, the photo-sensitive member is deteriorated by the wasteful laser beam irradiation to the photo-sensitive member.
In order to attain stable light intensity and provide an inexpensive recorder, the assignee of the present invention proposed the apparatus disclosed in U.S. Pat. No. 4,201,994, U.S. Pat. No. 4,443,695, U.S. Pat. No. 4,695,714, and U.S. Ser. No. 149,526 (filed on Jan. 28, 1988). However, further improvement has been desired.